1. Technical Field
This disclosure relates to devices having frequency dependent I/Q imbalance estimation, and to corresponding methods, software and integrated circuits.
2. Technical Field
RF receivers or transmitters of any type typically have mixers driven by a local oscillator to generate I and Q signals in quadrature, i.e., 90 degrees out of phase with each other with the result that there is no correlation between the two signals. As explained in US patent application 2003206603, it is well known in the art that deviations from the ideal I and Q signals can occur in the form of gain or magnitude imbalances, and relative phase imbalances. This can cause an unwanted image in a negative frequency part of a spectrum of the received signal, which can lead to interference or errors in subsequent processing stages such as demodulation, or filtering or algorithms to extract wanted signal components.
The phase imbalance can arise from local oscillator (LO) input signals to mixers not being exactly 90 degrees out of phase, or from path length differences, or errors caused by unbalanced parasitic capacitance, for example. Prior attempts to calibrate and correct for magnitude and phase imbalances have involved applying dedicated calibration signals at the receiver, for example, by switching the local oscillator to generate in-phase signals rather than out-of-phase signals. The resulting signals are analyzed to produce correction factors to be applied to received signals carrying data of interest, either at the analogue side or the digital side. R. A. Green, in “An Optimized Multi-tone Calibration Signal for Quadrature Receiver Communication Systems,” 10th IEEE Workshop on Statistical Signal and Array Processing, pp. 664-667, Pocono Manor, Pa., August 2000, shows an optimized multi-tone calibration signal to which linear regression techniques are applied to generate correction factors to update adaptive filters that are intended to compensate for gain and phase imbalances. Special circuitry typically needs to be used to produce, analyze, and correct for the results of analysis on such calibration signals. A further drawback is that the quadrature receiver typically cannot continue to actively receive normal transmitted data while the calibration is occurring.
US patent application 2003/206603 tries to provide I/Q calibration at a quadrature receiver that does not require that a separate calibration signal be transmitted to the receiver and that does not necessarily involve additional analogue components prior to the analogue to digital converters (ADCs). It achieves this by having an initial calibration period during which the local oscillator outputs are switched to feed in-phase signals to both mixers. Two switches (204, 206) and two phase shifters (208, 210) as well as the oscillator itself (280) are arranged to provide either (a) two LO output signals that are 90 degrees out of phase or (b) two LO output signals that are in phase (0 degrees out of phase). Mode (a) is used for normal operation of the receiver and mode (b) is used for the calibration period. One disadvantage of this is that the receiver cannot be used while it is being calibrated. I/Q imbalance factors in terms of phase and amplitude, are determined at different frequency bands, during the calibration period, using either frequency domain or time domain separation of frequency bands. These are then used to correct the I/Q imbalance when receiving signals in normal operation of the radio, with the LO signals 90 degrees out of phase.
One embodiment, shown in FIG. 9 of US2003/206603, is said to be capable of performing I/Q calibration and compensation and does not require a special switched local oscillator LO arrangement for an initial calibration period, for frequency independent imbalance estimation. However, it states that for frequency dependent phase mismatch (FDPM), the system does need the initial calibration period with the LO being switched to provide signals of equivalent phase to the mixers.
U.S. Pat. No. 5,872,538 (Fowler) shows frequency dependent I/Q imbalance correction, and shows a method of implementing it using a single complex FFT. Correction factors are obtained using a prior calibration process.